Page 30 - Hybrid-Renewable Energy Systems in Microgrids

Hybrid PV–wind renewable energy sources for microgrid application: an overview 15

carried out in Ref. [49] to investigate the technical challenges for frequency regulation
based on increased penetration of RES in a MG. It is found that frequency regulation
by diesel generator is possible, but the rapid acceleration and deceleration to support
the frequent fluctuations can shorten the life of the diesel generator. A BSS can replace
the diesel generator mainly in an emergency condition because of the quick response
of the battery compared to the conventional diesel generator. A study in Ref. [50]
considered that voltage stability support during a linear/nonlinear loading condition
is controlled by a load side converter, which supports the reactive power flow from
the battery in case reactive power demands increase from the nonlinear unbalanced
loads. If the storage capacity is not high enough to store all the surplus energy from the
renewable sources, load curtailment or using the dump load is a very common practice
to maintain steady-state operation of the system.
Integrating RES in a power system, whether in a main grid or MG, power qual-
ity must be maintained according to the IEC standards. Sensitive loads in a system
can be greatly affected by the voltage sags or swells. Voltage sags/swells can be
the result of sudden load increment/rejection or peak/low generations from the
RES. Because RES comes with the use of power electronic devices, which are a
great source of unwanted harmonics, they affect the system. Therefore harmonic
distortion needs to be considered during the designing of the system, and proper
arrangements need to be made to reduce the harmonics by using voltage source
converter control, incorporating LC filter, or installing a Flexible AC transmission
(FACTS) device.
MG in a grid-connected mode can be significantly affected by the transient faults.
Island operation is preferred to guarantee the stable operation of MG during fault
period by avoiding outage of the system for a long time and maintaining healthiness
of the system. Load shedding may need to be adopted in case the system starts to
oscillate after the fault clearance. Transient faults in a standalone MG can destabi-
lize the whole system, as the system dynamic is very poor in an off-grid MG and
therefore should have enough protection system to avoid damaging the equipment.

7 Simulation

To evaluate the benefit of PV–wind hybrid configuration, a small islanded MG is
considered. The conventional generation is rated to supply the entire load if RES is
not available at certain time of the day to ensure the reliability of the system. Transient
stability of the MG is analyzed in terms of the single RES and hybrid RES configura-
tion. The MG consists of a conventional generator, PV, wind, and a load. The basic
model of PV and wind used in this study is available in the Simulink Examples library
(power_4PVarray_400kW and power_wind_dfig_avg). The basic parameter of the
system is given in Table 1.4 (Fig. 1.6).
Three case studies have been carried out in this analysis. Three-phase to
ground fault is applied near the load terminal, and voltage at different terminals
of the network is analyzed when PV only, Wind only, and PV–wind are connected
to the MG.